from Wiktionary, Creative Commons Attribution/Share-Alike License

Examples

"boo hoo she was being mean so i punched her in the face and its her fault not mine she shouldnt have said mean things to me so in result i threw a fist instead of walking away like a person would normally do..boo hoo! its all her fault!"

I remember the first time one of my dearest friends referred to me as “boo”. That had to be way back in the late 80’s early 90’s. Today everyone knows what boo means. It’s a term of endearment, it’s a way of letting someone know that you like them. For example if I run into you at the mall and I shout out “Hey Boo!” You know I’m saying, “ glad to see you, you’re looking good and I like you” and a bunch of other good stuff all wrapped up into one neat little 3-letter package – B-O-O!...Remember Mo’Nique when she was on the Parker’s? Whenever her character Nikki Parker saw Professor Stanley Oglevee she would say, “Hey Boo!”

Another meaning for boo is boyfriend/girlfriend. From "My Boo" written by Jermaine Dupri, Manuel Seal, Jr., Adonis Shropshire, Alicia Keys, and Usher: "Know that you're that one for me/it's clear for everyone to see/Ooh baby, you will always be my boo" http://en.wikipedia.org/wiki/My_Boo_(Usher_and_Alicia_Keys_song)

Supposedly derived from Boh, a general from Norse myth. Boh was a son of Odin and a general, and was so fierce and powerful that the mere mention of his name struck fear in the hearts of his enemies. Viking warriors would shout his name when launching a surprise attack, and that eventually led to the custom of shouting "Boo!" when surprising someone.